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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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  • ABL - Abbreviation for atmospheric boundary layer.

  • absolute - 1. With respect to atmospheric motions, refers to an absolute coordinate system; for example, absolute vorticity as distinguished from relative vorticity. 2. In climatology, the highest or lowest recorded value of a meteorological element, whether at a single station or over an area, during a given period. It is most frequently applied to extremes of temperature: absolute maximum is the highest recorded shade temperature; absolute minimum is the lowest recorded. 3. See Kelvin temperature scale.

  • accumulation - 1. See snow accumulation. 2. In glaciology, the quantity of snow or other solid form of water added to a glacier or snowfield by alimentation; the opposite of ablation. Compare snowpack.

  • accuracy - The extent to which results of a calculation or the readings of an instrument approach the true values of the calculated or measured quantities. Compare precision.

  • activity - Measure of the effective concentration of a chemical in the gas or liquid phase. The activity is usually less than the mass concentration, from which it differs due to the nonideal nature of gases and solutions. In the limit of very dilute mixtures, the activity is the same as the concentration for liquid solutions, or the partial pressure for gases. The activity coefficient, usually denoted by g[γ] or f, is the ratio of the activity to the actual physical concentration.

  • adiabat - In most contexts, same as dry adiabat. See also saturation adiabat, pseudoadiabat.

  • adiabatic - See adiabatic process.

  • advection - The process of transport of an atmospheric property solely by the mass motion (velocity field) of the atmosphere; also, the rate of change of the value of the advected property at a given point. Advection may be expressed in vector notation by -u · Ñ[∇]f[&phgr;], where u is the wind vector, f[&phgr;] the atmospheric property, and Ñ[∇]f[&phgr;] the gradient of the property. In three-dimensional Cartesian coordinates, it is where u, v, and w are the wind components in the eastward, northward, and vertically upward directions, respectively. The first two terms compose the horizontal advection and the last term is the vertical advection. Also, it should be noted that the property f[&phgr;] may itself be a vector field. Often, particularly in synoptic meteorology, advection refers only to the horizontal or isobaric components of motion, that is, the wind field as shown on a synoptic chart. Regarding the general distinction (in meteorology) between advection and convection, the former describes the predominantly horizontal, large-scale motions of the atmosphere, while convection describes the predominantly vertical, locally induced motions. In oceanography, advection refers to the horizontal or vertical flow of seawater as a current.

  • aerosol - A colloidal system in which the dispersed phase is composed of either solid or liquid particles, and in which the dispersion medium is some gas, usually air. There is no clear-cut upper limit to the size of particles composing the dispersed phase in an aerosol, but as in all other colloidal systems, it is rather commonly set at 1 m[&mgr;]m. Haze, most smokes, and some fogs and clouds may thus be regarded as aerosols. However, it is not good usage to apply the term to ordinary clouds with drops so large as to rule out the usual concept of colloidal stability. It is also poor usage to apply the term to the dispersed particles alone; an aerosol is a system of dispersed phase and dispersing medium taken together. Compare airborne particulates, particles, PM-2.5, PM-10.

  • age - In oceanography, the time interval between an astronomical event and the corresponding tidal phenomenon. For example, the old oceanographic term for the lag in days between the occurrence of syzygy and the highest spring tide is the "age of the tide;" the lag between perigee and the highest perigean tide is the "age of parallax inequality."

  • air - Mixture of gases forming the earth's atmosphere, consisting of nitrogen (~78 percent), oxygen (~21 percent), water vapor, and other trace gases such as carbon dioxide, helium, argon, ozone, or various pollutants. The concentration of water vapor is very variable, being a strong function of temperature and, hence, altitude in the atmosphere. Dry air is referred to as air from which measurable amounts of water vapor have been physically removed. Pure, dry air has a density of 1.293 kg m-3 at a temperature of 273 K and a pressure of 101.325 kPa. Apart from the variability of water vapor, the composition of air is essentially constant to an altitude of at least 50 km and is presently approximated as follows:

    ConstituentSymbolVolume Fraction
    Oxygen O2 0.20948
    Argon Ar 9.3 X 10-3
    Carbon dioxide CO2 3.6 X 10-4
    Neon Ne 1.82 X 10-5
    Helium He 5.24 X 10-6
    Methane CH4 1.7 X 10-6
    Krypton Kr 1.14 X 10-6
    Hydrogen H2 5.0 X 10-7
    Nitrous oxide N2O 3.3 X 10-7

    The concentration of ozone is variable, between 10 and 0.1 parts per million. Carbon dioxide, methane, and nitrous oxide have all been increasing since the beginning of the industrial age.

  • air mass - 1. A widespread body of air, the properties of which can be identified as 1) having been established while that air was situated over a particular region of the earth's surface (airmass source region), and 2) undergoing specific modifications while in transit away from the source region. An air mass is often defined as a widespread body of air that is approximately homogeneous in its horizontal extent, particularly with reference to temperature and moisture distribution; in addition, the vertical temperature and moisture variations are approximately the same over its horizontal extent. The stagnation or long-continued motion of air over a source region permits the vertical temperature and moisture distribution of the air to reach relative equilibrium with the underlying surface. See airmass classification. 2. In radiation, the ratio of the actual path length taken by the direct solar beam to the analogous path when the sun is overhead from the top of the atmosphere to the surface. Extrapolation of surface measurements to zero air mass was the original method for estimating the value of solar irradiance at the top of the atmosphere. 3. See optical air mass.

  • air parcel - An imaginary volume of air to which may be assigned any or all of the basic dynamic and thermodynamic properties of atmospheric air. A parcel is large enough to contain a very great number of molecules, but small enough so that the properties assigned to it are approximately uniform within it and so that its motions with respect to the surrounding atmosphere do not induce marked compensatory movements. It cannot be given precise numerical definition, but a cubic foot of air might fit well into most contexts where air parcels are discussed, particularly those related to static stability. Any fluid parcel may be defined similarly.

  • air temperature - The temperature indicated by a thermometer exposed to the air in a place sheltered from direct solar radiation.

  • airmass classification - A system used to identify and to characterize the different air masses according to a basic scheme. A number of systems have been proposed, but the Bergeron classification has been the most widely accepted. In this system, air masses are designated first according to the thermal properties of their source regions: tropical (T); polar (P); and less frequently, arctic or antarctic (A). For characterizing the moisture distribution, air masses are distinguished as to continental (c) and maritime (m) source regions. Further classification according to whether the air is cold (k) or warm (w) relative to the surface over which it is moving indicates the low-level stability conditions of the air, the type of modification from below, and is also related to the weather occurring within the air mass. This outline of classification yields the following identifiers for air masses: cTk, cTw, mTk, mTw, cPk, cPw, mPk, mPw, cAk, mAk, mAw; the last of which is never used. H. C. Willett, in his classification, introduces further distinction between stable (s) and unstable (u) conditions in upper levels. Some authors may include equatorial (E), monsoon (M), or superior air (S) in their classifications. Others prefer to omit the arctic (A) type and describe all air masses on the basis of polar and tropical air, separated by the polar front.

  • Aleutian low - The low pressure center located near the Aleutian Islands on mean charts of sea level pressure. It represents one of the main centers of action in the atmospheric circulation of the Northern Hemisphere. The Aleutian low is most intense in the winter months; in summer it is displaced toward the North Pole and is almost nonexistent. On a daily basis, the area of the Aleutian low is marked by alternating high and low pressure centers, moving generally to the eastward; it is not the scene of an intense stationary low. Normally the depth of intensity of the low pressure areas exceeds the intensity of the high pressure areas, so that the region is one of low pressure on the average. The travelling cyclones of subpolar latitudes usually reach maximum intensity in the area of the Aleutian low. The Aleutian low and its counterpart in the Atlantic Ocean, the Icelandic low, compose the Northern Hemisphere's subpolar low pressure belt.

  • ambient air - 1. Background, environmental, or surrounding air. When studying the dynamic and thermodynamic processes acting on an individual element such as an air parcel, cloud, smoke plume, raindrop, or ice crystal, ambient air represents the atmosphere outside of that element. The ambient air is often assumed to be static and of relatively large domain, within which the element resides. 2. The air that surrounds us, within which we live. When air pollutants of high concentration from exhaust or stack gases are emitted into cleaner air, the resulting polluted mixture is called the ambient air. National Ambient Air Quality Standards (NAAQS) apply to this final mixture, not to the undiluted emission gases.

  • ambient temperature - The temperature that is characteristic of the atmosphere surrounding a small-scale feature such as a cumulus cloud.

  • amplitude - Often the greatest magnitude at a given point of any spatially and temporally varying physical quantity governed by a wave equation; can also mean the spatial part of a time-harmonic wave function. For example, in the time-harmonic (or sinusoidal) scalar wave function with circular frequency w[&ohgr;], y[&psgr;](x, t) = f[&phgr;](x) exp(-iw[&ohgr;]t), where f[&phgr;](x) is the (complex) amplitude of the wave, although the modulus of f[&phgr;] also may be called its amplitude. The (complex) amplitude of the scalar plane harmonic wave y[&psgr;](x, t) = A exp(ikx - iw[&ohgr;]t - iq[&thgr;]) with wavenumber k and initial phase q[&thgr;] is A exp(ikx - iq[&thgr;]), the modulus of which, |A|, is also called the amplitude of the wave. In its most general sense, amplitude means extent or size. Thus the amplitude of a wave is some measure of its size.

  • analysis - 1. In synoptic meteorology, a detailed study of the state of the atmosphere based on observations, usually including a separation of the entity into its component patterns and involving the drawing of families of isopleths for various elements. Thus, the analysis of synoptic charts may consist, for example, of the drawing and the interpretation of the patterns of wind, pressure, pressure change, temperature, humidity, clouds, and hydrometeors, all based on observations taken or forecast simultaneously. 2. A procedure to project the state of the atmosphere (or any system) as known from a finite set of imperfect, irregularly distributed observations onto a regular grid or to represent the atmospheric state by the amplitude of standard mathematical functions. The grid and/or functional expansion allows for subsequent forecasts by numerical integration or for easy diagnostic study. An analysis may be looked upon as a space-time interpolation system. Essential ingredients of analysis are a background field, usually a short-range forecast although persistence and climatology are options, a forecast model, knowledge of the relative error of the many different observational platforms employed as well as knowledge of the error in the background field, and the spatial covariance of the various errors. Real time computational considerations generally limit the complexity of methods used. Historically important is the subjective hand analysis, that is, drawing isobars on a map given mean sea level pressure at a set of observing stations. Most analysis systems in the past have been at best 3D spatial analyses; time-space analysis systems have recently emerged at operational prediction centers. A long series of analyses is of great utility for studying the behavior of the atmospheric climate system. See reanalysis, variational objective analysis.

  • anticyclonic - Having a sense of rotation about the local vertical opposite to that of the earth's rotation; that is, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere, undefined at the equator. It is the opposite of cyclonic.

  • AP - Abbreviation for anomalous propagation.

  • aridity - The degree to which a climate lacks effective, life-promoting moisture; the opposite of humidity, in the climate sense of the term. The overall concept of aridity versus humidity is coming to be known as precipitation effectiveness. Two basic approaches have been made. The first, used by W. Köppen and modified by Bailey, does not openly define aridity, but rather assigns delimiting values of annual precipitation (treated with regard to distribution and temperature) to separate a dry climate from other types. The second approach actually prescribes a measure of aridity or precipitation effectiveness and uses these values as a primary parameter of classification. Of this type are Thornthwaite's precipitation-effectiveness index and moisture index, E. de Martonne's index of aridity, W. Gorczyski's aridity coefficient, Lang's moisture factor, and Ångström's humidity coefficient.

  • ash - 1. Airborne particulates produced as a combustion product. These affect air quality, and their emission and ambient concentration is often regulated. For example, coal ash can consist of oxides of silicon, aluminum, iron, calcium, magnesium, sodium, potassium, sulfur, and titanium. 2. Airborne particulates produced by a volcano. These are often fine particles of rock with very sharp edges, which cause great abrasion and wear when ingested in the air intake of an engine, and can quickly lead to engine failure.

  • aspect - The compass direction toward which a land slope faces. The direction is taken downslope and normal to the contours of elevation.

  • atmosphere - 1. A gaseous envelope gravitationally bound to a celestial body (e.g., a planet, its satellite, or a star). Different atmospheres have very different properties. For instance, the atmosphere of Venus is very thick and cloudy, and is responsible for producing the very high surface temperatures on that planet by virtue of its greenhouse effect. On the other hand, the Martian atmosphere is very sparse. Earth's atmosphere is intermediate between these two extremes. It is distinguished from all other known atmospheres by its very active hydrologic cycle. One need merely examine pictures of Earth from space to appreciate the intricate cloud structures. Water in Earth's atmosphere plays a very important energetic role. Because of its chemical composition, most incoming sunlight passes through Earth's atmosphere and is absorbed at the ground. This heat is transported to the atmosphere through sensible heat and moisture fluxes. Upon condensation, this heat is then released into the atmosphere. The thermodynamics of water vapor is the crucial factor to the existence of severe storms in Earth's atmosphere. Since more solar radiation is absorbed in the Tropics than at high latitudes, the atmosphere (and the ocean) transports heat poleward. These motions are heavily altered by the effects of planetary rotation to determine the atmospheric general circulation. Fluid dynamical instabilities play a large role in this circulation and are crucial in determining the fluctuations in this circulation that we call "weather."The atmosphere may be conceptually divided into several layers, according to its thermal and ionization structure. The region where the temperature decreases because of the upward heat flux is called the troposphere. Above it, there is a layer in which temperature increases upward because of ozone absorption of solar radiation, the stratosphere. Above this, the temperature decreases in the mesosphere, and above this, in the thermosphere, the extremely energetic radiation causes temperature to increase with height out to the outer reaches of Earth's atmosphere, the exosphere. Within the mesosphere and thermosphere, solar radiation is sufficiently energetic to ionize gases. This produces the ionosphere. 2. As a unit of pressure, see standard atmosphere. 3. See standard atmosphere, model atmosphere.

  • atmospheric turbulence - See turbulence.

  • available potential energy - That portion of the total potential energy that may be converted to kinetic energy in an adiabatically enclosed system. If the atmosphere were horizontally stratified, none of its potential energy would be so convertible. Thus, the available potential energy for the atmosphere in a given state is the difference between the value of the total potential energy in the given state and the value it would have after an adiabatic redistribution of mass had produced a horizontal stratification.

  • average - 1. Same as arithmetic mean. 2. In a very broad sense, any number lying between the extremes of a set of numbers. Compare Reynolds averaging, ensemble average, area average, mean, ergodic.

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